Photovoltaic devices provide clean, quiet and reliable sources of electrical power and because of shortages and environmental problems associated with fossil and nuclear fuels and because of recent advances in technology which have significantly decreased the cost and increased the efficiencies of photovoltaic devices, photovoltaic power is of growing commercial importance. Initially photovoltaic devices were manufactured from single crystalline material. These devices were expensive, delicate, fairly bulky and difficult to manufacture in fairly large area configurations. Various techniques have now been developed for preparing thin film semiconductor materials which manifest electrical properties which are equivalent, and in many instances superior to, their single crystalline counterparts. These thin film materials may be readily deposited over very large areas and on a variety of substrates. Such alloys and techniques for their preparation are disclosed, for example, in U.S. Pat. Nos. 4,226,898 and 4,217,374. One important class of photovoltaic devices comprise a layer of intrinsic semiconductor interposed between two oppositely doped semiconductor layers. Such devices are termed P-I-N or N-I-P devices depending on the order of the layers and the two terms shall be used interchangeably herein.
Glow discharge deposition comprises one particularly important class of techniques for the preparation of thin film semiconductor materials. In a glow discharge method a process gas, typically at subatmospheric pressures, is energized by an electrical field so as to produce a plasma comprised of ionized and/or otherwise activated species derived from the process gas. The plasma acts to produce a semiconductor deposit on a substrate maintained in proximity thereto. Initially, such glow discharge deposition processes were energized by direct current, or, more commonly, by alternating current in the radio frequency range. While such techniques produce high quality semiconductor materials, deposition rates obtained thereby are fairly low and significant amounts of process gas are utilized non-productively. Attempts to raise the deposition rate either by increasing the gas pressure or by greatly increasing the power density results in the production of polymeric and oligomeric species which contaminate and degrade the semiconductor layers.
It has been found that microwave energy may be beneficially employed to energize a plasma in a glow discharge deposition process and that a microwave energized plasma process is particularly advantageous for semiconductor fabrication since very high rates of deposition may be achieved concomitant with a greatly enhanced process gas utilization. The application of microwave energy to glow discharge semiconductor depositions is disclosed in U.S. Pat. No. 4,517,223.
While microwave energized processes are attractive because of their high deposition rates and high rates of gas utilization, it has been found that the semiconductor materials deposited thereby are generally of somewhat lower quality than those materials derived from a RF or DC energized plasma. Photovoltaic devices having microwave-produced semiconductor layers have an overall efficiency which is generally lower than that of corresponding RF prepared devices.
Heretofore it has been generally believed that a positive bias should be applied to a microwave generated plasma to enhance the deposition of semiconductor layers. By bias is meant that a charged wire or other electrical conductor is placed in the plasma and functions to repel particularly charged species toward the substrate during the course of the deposition. As disclosed, for example, in U.S. Pat. No. 4,379,943 a bias of approximately 80 volts is applied to a deposition plasma to increase the bombardment of the substrate by positive ions and increased deposition rate and material quality are attributed thereto.
The prior art has applied a fairly high bias to a microwave energized plasma for the entirety of the deposition process and conventional wisdom teaches that such bias is necessary to improve semiconductor material quality. In accord with the present invention, it has been found that increased positive ion bombardment, attendant upon high plasma bias can actually be detrimental to the quality of semiconductor material deposited in a glow discharge process.
Specifically, it has been found that a higher quality of semiconductor material of photovoltaic devices is produced in a microwave energized glow discharge process where positive ion bombardment of the substrate is at least partially limited. Such limitation is accomplished by significantly decreasing or eliminating any positive bias in the plasma or by taking the affirmative step of placing a screening electrode in the plasma to limit positive ion bombardment. It has been found that photovoltaic devices manufactured from semiconductor material produced in this manner have a fill factor which is better than that of devices made from material in which positive ion bombardment is not limited. Fill factor is a measure of photovoltaic device performance determined by plotting the voltage-current curve of the device under illumination and by taking the ratio of the area under the curve versus the area of the corresponding rectangle defined by extending lines normal to the short circuit current and open circuit voltage points on the axes. The higher the fill factor the better the efficiency of the device. While the fill factor of devices made in this manner is generally improved, it has been found that the open circuit voltage of such cells is low.
In accord with the present invention, it has been found that in a microwave deposition process for the manufacture of photovoltaic devices, the open circuit voltage may be increased and the improved fill factor retained, if the plasma bias is appropriately manipulated during a portion of the time during which various of the semiconductor layers are being deposited. Specifically, it has been found that in the preparation of an N-I-P type photovoltaic device, imposition of a high positive bias on a microwave generated plasma only during the deposition of that portion of the intrinsic layer which is proximate one of the doped layers, will improve the cells' open circuit voltage. This improvement occurs concomitant with the broadening of the band gap of the material deposited under the highly positive bias and is believed to be a result of incorporation of additional hydrogen into the semiconductor material.
It has been known to vary the band gap of the intrinsic layer of a photovoltaic device for various purposes; however, in the prior art, such band gap variation is achieved by the addition of various band gap modifying elements to the deposition gas. For example, it has been found that the addition of germanium to a silicon based semiconductor material will narrow the band gap thereof. Similarly, the addition of fluorine to a silicon based material will broaden the band gap. By appropriately changing gas ratios during the deposition, various gap configurations may be achieved. U.S. Pat. No. 4,471,155 discloses a P-I-N type photovoltaic device in which the portion of the intrinsic layer proximate one or both of the doped layer is made of a wider band gap material for purposes of increasing cell voltage. As disclosed therein, a main portion of the intrinsic layer is made of a relatively narrow-gap, germanium-containing material and the interface region is devoid of germanium and has a wider gap. This cell is particularly used in multiple, stacked cell configurations.
U.S. Pat. No. 4,379,943 discloses a P-I-N type cell in which a first doped-intrinsic interface is comprised of a silicon-hydrogen material having a relatively narrow gap and the remainder of the intrinsic body is a silicon-hydrogen-fluorine alloy of wider band gap. This particular structure is used to prevent fluorine etching of the first interface during deposition. U.S. Pat. No. 4,817,082 discloses a graded gap structure in which band gap varies from a fairly wide gap at the doped-intrinsic interface to a narrower gap at an intermediate portion, these variations are resultant from compositional grading within the device. Also disclosed in the '082 patent is the use of a pure hydrogen plasma to passivate interface states in the first doped layer prior to the deposition of the intrinsic layer thereupon. U.S. Pat. No. 4,547,621 also discloses the use of composition variation to enlarge the gap of a photovoltaic device of the P-I-N type, proximate one of the doped-intrinsic interfaces.
The prior art does not recognize that the band gap of a semiconductor alloy, and hence the open circuit voltage of a photovoltaic device including that alloy, may be readily controlled in a microwave energized process by control of plasma bias and hence positive ion bombardment. The present invention provides a simple, easy to apply method by which the hydrogen content of a silicon alloy material may be readily controlled by controlling the bias of the deposition plasma. This control allows for manipulation of the band gap of the material and may be used to manufacture various graded structures. In accord with the present invention, the method may be utilized to manufacture P-I-N or N-I-P type photovoltaic devices having an improved fill factor as well as a good open circuit voltage. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow: